This invention relates to a magnetic tape cassette, and more specifically to a mechanism for preventing ingress of dust from the outside into a video tape cassette.
A typical prior art arrangement will first be explained with reference to FIGS. 1 to 3 illustrating the present invention. Conventionally, a magnetic tape cassette, notably a video cassette housing for 8 mm video tape, comprises, as shown in FIG. 1, a front lid 11, inner lid 12, upper half housing part 13, lower half housing part 16, and reels 14, 14'. The front lid 11 is connected to the upper half housing part 13, with a pair of pivots 3, 3,' on the lid engaged with corresponding openings 4, 4' formed in the housing part, imparting some freedom of turning to the lid 11 which is normally urged to the closing position with a spring on one of the pivot3, 3'. The front lid 11 has openings 2, 2' adapted to engage pivots 1, 1' of the inner lid 12, whereby the turning motion of the front lid 11 is transmitted to the inner lid 12, allowing the latter to rotate relative to the former. Thus, when the two lids are in the closing position they protect the magnetic tape portion held there-between, and as the front lid turns upward, the inner lid also turns open to expose the tape. The upper and lower half housing parts 13, 16 are securely joined, with holes and hollow columns 5, 5', 15, 15' of the lower part mated with internally threaded holes 6, 6', 7, 7' of the upper part by screws. The upper and lower half housing parts 13, 16 are formed with pairs of guide groove walls 8, 8' (8' being not shown in FIG. 1), and 9, 9', respectively, so that when the two half parts are mated together a pair of guide grooves result. Inner lid guide pins 10, 10' are fitted, respectively, in the guide groove 17 (shown in FIG. 2) defined by the guide groove walls 8, 9 and in the guide groove 17' defined by the guide groove walls 8', 9' (FIG. 2). The construction described enables the inner lid guide pins 10, 10' to slide along the guide grooves 17, 17' with the rotation of the front lid 11 as illustrated in FIG. 2. FIG. 3 shows the relative positiions of the front lid 11, inner lid 12, and inner lid guide pin 10', when the front lid is closed.
In the conventional magnetic tape cassette, especially video tape cassette, of such design, as can be seen from FIG. 3, the finally closed inner lid position with the front lid closed is governed by the position of the inner lid guide pin 10', and the posiiton of the pin 10', in turn, is dictated by the guide groove 17'. Long-time use of the cassette designed in this way or the dimensional tolerance for the cassette will add to the play between the guide groove 17' and the inner lid guide pin 10'. Eventually an excessive play will result between the inner lid pivot 1' and the front lid opening. This can produce a gap at 20 where the front lid 11 and inner lid 12 normally are in contact when the front lid 11 is in the closed position. The gap then admits dust from the outside into the cassette housing, and the dust deposit on the magnetic tape can result in deteriorated video picture quality.